Wireless communication systems are required to transmit ever-increasing amounts of data, in support of expanded subscriber services, such as messaging, e-mail, music and video streaming, and the like. Transmitting a higher volume of data over a given channel requires transmission at a higher data rate.
One known technique to improve data transmission rates in wireless communications is the use of multiple input, multiple output (MIMO) technology, wherein signals are transmitted from multiple transmit antennas and may be received by multiple receiver antennas. Using advanced coding and modulation schemes, two or more streams of data may be transmitted simultaneously to a receiver, increasing the data rate.
Maintaining high data rates in MIMO systems requires fast link adaptation. That is, the transmitter must constantly alter its selection of transmission parameters, such as the modulation and coding scheme selected, or antenna configuration, based on the current characteristics of the channel, which can change rapidly. In a Frequency Division Duplex (FDD) system, the instantaneous downlink channel conditions are not available at the base station, and must be determined by a receiver and communicated to the base station. In Wideband CDMA (WCDMA) and Long Term Extension (LTE), the instantaneous downlink channel conditions are communicated to the base station through a Channel Quality Indicator (CQI).
Estimating the CQI is a delicate task, which involves a calculation of a signal to interference and noise ratio (SINR). For a multi-stream transmission, estimating the SINR is a more complex task than estimating the SINR for a single-stream transmission, due to the inter-stream interference. Traditional SINR estimation techniques involve summing the noise and respective inter-stream interferences (a matrix quantity) to obtain an impairments matrix, and then inverting the matrix to calculate an inverse impairments matrix. The impairments matrix is of size nrx×nrx, where nrx is the number of receive antennas. Matrix inversion is a computationally intensive mathematical operation. The limited computing resources (and power budget) of a mobile receiver restrict the frequency with which the CQI can be calculated and communicated to a base station, thus reducing the link adaptation rate, which places an upper bound on the available data rate.